The present invention relates to a pressure sensor, especially a differential pressure sensor, which includes a mounting structure and has an inner region comprised of a membrane which may be acted on by fluid pressure applied to both sides.
Pressure or differential pressure sensors having a diaphragm with measurement electrical elements located thereon are disclosed by Jensen, et al. in U.S. Pat. Nos. 6,030,709 and 6,085,596 (Jensen '709 and Jensen '596, hereinafter); and by Krog, et al. in U.S. Pat. No. 7,010,984 (Krog '984, hereinafter) The diaphragm is formed as a thinned region of a semiconductor device, a sensor die, which is sealingly clamped or bonded into a two-part mounting. The measurement element on the membrane may be connected to further electronic components arranged on a circuit board fastened or supported on a part of the mounting.
Through-holes are formed in each of the two parts of the mounting, in order to impinge the sensor diaphragm with one or more test fluids under pressure. The test fluids may be liquids, gasses or a gas and a fluid. Fluid may be ducted to the sensor diaphragm from two portions of a fluid circuit, thereby measuring a differential pressure. One side of the diaphragm may be ducted to a calibration fluid pressure source, such as the atmosphere, and the other to a test fluid source, in which case the pressure sensor measures the “absolute” pressure of the test fluid relative to the standard.
Pressure sensors may be required to operate in especially difficult chemical environments wherein the test fluids are highly corrosive. Jensen '709 and Jensen '596 disclose pressure sensors that are fabricated as a plurality of sensor die sites on single crystal silicon wafers using microelectronic materials and manufacturing processes. A thinned region that carries pressure sensitive resistors is achieved by orientation dependent etching (ODE) of the silicon at the wafer level. A dual layer coating comprised of an electrically insulating film and a deposition of an amorphous metal layer are applied to both sides of the sensor die wafer as last process stages before the wafers are singularized (i.e., separated into single wafers), i.e. by dicing.
While the wafer scale fabrication of sensor die disclosed in Jensen '709 and Jensen '596 is effective in producing sophisticated core sensor transducer elements, the ultimate cost of a finished sensor assembly is importantly affected by the design and implementation of a system of seals to confine test fluids to impinge only protected surfaces of the sensor die and inert materials in the sensor assembly. Microelectronic processing technology is such that the active area of a pressure sensor may be made quite small, less than 1 mm2 per sensor die site. Electrical output pads for connecting the sensor to an outside system require some additional die site area. After improving these two factors, a next most important sensor design feature affecting die cost is the die area that must be provided for sealing “off” the unprotected portions of the sensor die, electrical leads and mounting adhesives from corrosive test fluids.
Jensen '709 and '596 and Krog '984 disclose two sealing designs for providing die surface fluid seals: (1) mechanical sealing members such as an O-ring and (2) an adhesive layer applied in an undisclosed fashion and cured after assembly. Mechanical sealing members require substantial sensor die surface area to ensure that an adequate area of seal will be formed given the variations that are expected in the amount of seal compression due to component and assembly dimensional variations. Adhesive sealing between two closely spaced components is difficult to control in the case wherein a perimeter seal is to be formed around a central surface region that must remain uncoated by the adhesive during the adhesive cure process.
The potential reliability or manufacturing yield difficulties of both of the above two die surface sealing designs may be managed by adding sealing area to the die site design. However, adding die site area for sealing reliability increases sensor cost in several ways: fewer die per wafer, lower yields due to defects in corrosion protection layers, and larger, more rigid mounting components are needed to protect more fragile individual sensor die.
There is a need, therefore, for a pressure sensor sealing design that minimizes the sealing area needed at the sensor die surface level. A spatially efficient sealing design and fabrication method will allow reduced sensor die area, thereby reducing the die surface area requiring full integrity of protective coatings such as the amorphous metal films disclosed by Jensen '709 and Jensen '596, and improving the yield of good die sites per wafer. Producing more sensor die per wafer will also directly lower cost. Finally, die having smaller overall dimensions relative to thinned diaphragm areas will experience less strain and strain induced cracking during sensor assembly and while in service by the end user.